A method for the highly accurate quantification of gas streams by on-line chromatography

Abstract

Quantification of the gas streams from chemical systems such as catalytic reactors are routinely performed by on-line chromatography. Gas chromatographs used for this purpose are typically provided with a combination of thermal conductivity (TCD) and flame ionization (FID) detectors to be able to detect and quantify both permanent gases and hydrocarbons. However, the accuracy of the quantification is hindered by the intrinsic limitations of each type of detector. Namely, TCD has low sensitivity and FID does not detect permanent gases; COx, N2, H2, etc. Therefore, modern gas chromatographs include methanizer units to partially overcome this shortcoming by converting COx to methane. However, as far as these authors know, the literature has not presented an analytical method to characterize gas streams with high accuracy by the simultaneous use of a combination of a TCD-FID detection system provided with a methanizer. In this work, we developed analytical methods for this purpose. Our approach consisted on formulating a mathematical model for the well-known external and internal standard quantification methods in gas chromatography. We specifically applied the methodology to the analysis of the gas streams from a catalytic reactor performing the combustion of methane. We found that the commonly applied external standard method leads not only to inaccurate quantification but also to physically meaningless conclusions on the behavior of the selected model system. In contrast, the internal standard method led to a highly accurate quantification with a physical meaning. Considering these findings, our contribution also draws attention to the need for a thoughtful application of chromatographic methods when studying the reactivity of gas systems.

A Method for the Accurate Quantification of Gas Streams by Online Mass Spectrometry

“…For comparison purposes, a set of reaction tests were done in another reaction setup coupled online to a gas chromatograph. The specific details and the methods for quantification in the latter system are presented elsewhere …”

“…The catalytic reactor of the OXIDATEST was loaded and operated keeping the same conditions used for the tests made in the CATLAB reactor, except for the use of N 2 as both internal standard and balance gas instead of He and Ar. Further details on the OXIDATEST setup and on the GC quantification method are provided by Sandoval-Bohorquez et al…”

“…Therefore, the mass of the reactants entering the reactor equals the mass of the products at its outlet. Herein, this principle is applied by means of a metric called the mass balance coefficient (θ) that relates the masses of the gases that inlet and outlet the reactor, eq where is the molar weight of compound j . Of course, the maximum value of θ is 1 for this value corresponds to a full closure of the mass balance.…”

“…On the other hand, eq could also be applied to each element that participates in the reaction: carbon balance, oxygen balance, among others. Further details on how to apply mass balances to improve the accuracy of online quantification systems are presented elsewhere …”

“…Particularly, the analysis of mass balances was implemented as a control tool for checking both the accuracy and physical meaning of the developed method. This strategy has proved successful for improving the accuracy of online GC analysis but, as far as the authors of this paper know, it had not been used as part of a method for accurately quantifying gaseous streams from catalytic systems by mass spectrometry. Herein, the catalytic combustion of methane over a continuous plug-flow bed reactor was used as a model reaction to test the accuracy of the mathematical methods developed from both the internal and the external standard methods of calibration.…”